In recent years there has been a trend away from conventional sterilization techniques such as autoclaving or ethylene oxide treatment for sterilizing disposable equipments for hospital and medical use such as tubes, syringes, vials and the like. Rather, sterilization with ionizing radiation has evolved as a preferred method of sterilizing many types of disposable equipments for hospital use and biomedical applications. Sterilization of such articles is accomplished with a dosage of up to about 5 Mrads of gamma radiation, typically about 2.5 Mrads.
Ionizing radiation, however, can profoundly alter the molecular structure and macroscopic properties of polymeric materials such as polypropylene and propylene-ethylene copolymers that are used to manufacture disposable medical devices. In the case of polypropylene, and propylene-ethylene copolymers, exposure to ionizing radiation can result in deterioration of physical properties as evidenced by brittleness, discoloration, cracking and a loss of melt flow stability. Moreover, in the case of semicrystalline polypropylene or propylene-ethylene copolymers without an effective radiation resistant additive package, degradation of physical properties resulting from irradiation can cause a limited shelf life of the product. Consequently, there is a need for improved polypropylene blends for the production of tubes, syringes, vials and similar articles that would exhibit post-irradiation resistance to such degradation.
U.S. Pat. No. 4,888,369, issued Dec. 19, 1989 to Moore, discloses a high energy radiation resistant, polypropylene composition in which is dispersed a hindered amine component, a hindered phenolic component and a phosphorous containing component. U.S. Pat. No. 4,666,959, issued May 19, 1987 to Weissberger et al., discloses a propylene polymer molding composition which is resistant to discoloration and losses in physical properties upon sterilization by high energy radiation which comprises a blend of a propylene polymer with small amounts of a polymeric hindered amine, an alkyl phosphite and a specific phenolic antioxidant as additives.
U.S. Pat. No. 4,110,185, issued Aug. 29, 1978 to Williams et al., discloses a semicrystalline polymer article which is sterilized by radiation, the polymer having incorporated therein during the irradiation a mobilizing amount of a non-crystalline mobilizer. The preferred mobilizer is a liquid mobilizer which is not highly viscous, and in particular, a hydrocarbon oil or phthalic ester oil.
European Patent Application 90122882.5, published Dec. 6, 1991, discloses a radiation resistant polypropylene composition containing a polypropylene having a substantially syndiotactic structure and a phosphorus-containing anti-oxidant, an amine-containing anti-oxidant and/or a nucleating agent. Japanese Kokai Patent Application No. 2-292346 discloses a radiation resistant composition for nuclear reactors at 500 Mrad or higher containing at least 3 parts by weight of one of several antioxidants with a polyolefin resin. The foregoing references do not however, teach the radiation resistant polypropylene blend of the present invention.